Alkyl transfer of alkyl aromatics with group vi-b metals on boria-alumina

ABSTRACT

A PROCESS FOR THE ALKYL TRANSFER OF MONO-ALKYL AROMATICS INCLUDING CONTACTING A MONO-ALKYL AROMATIC FEED MATERIAL, PARTICULARLY TOULENE, WITH A CATALYST COMPRISING A GROUP VI-B METAL, SUCH AS CHROMIUM, MOLYBDENUM OR TUNGSTEN, AND BORIA DEPOSITED ON AN ALUMINA BASE, AT A TEMPERATURE BETWEEN ABOUT 800 TO 1100*F., A PRESSURE OF ABOUT 0 TO 2000 P.S.I.G., AND A LIQUID HOURLY SPACE VELOCITY OF ABOUT 0.1 TO 10, AND IN THE PRESENCE OF HYDROGEN INTRODEUCED AT A RATE OF ABOUT 1 TO 10 MOLES HYDROGEN PER MOLE OF HYDROCARBON FEED. PROMOTERS SELECTED FROM GROUP I, GROUP II, GROUP IV AND THE RARE EARTH METALS OF THE PERIODIC SYSTEM CAN ALSO BE ADDED TO THE CATALYST. DEACTIVATED CATALYSTS MAY BE PERIODICALLY REJUVENATED BY DISCONTINUING THE INTRODUCTION OF THE AROMATIC FEED MATERIAL AND PURGING WITH HYDROGEN AND THE CATALYST CAN BE, REACTIVATED BY CALCINATION IN AN ATMOSPHERE SUCH AS AIR. WHERE TOLUENE IS THE FEED MATERIAL, THE ALKYL TRANSFER PRODUCT MAY BE DISTILLED TO SEPARATE BENZENE, TOLUENE AND XYLENES, THE TOLUENES MAY BE RECYCLED TO THE ALKYL TRANSFER STEP, THE XYLENES MAY BE CRYSTALLIZED TO SEPARATE PARA-XYLENE FROM THE REMAINING XYLENES, THE MOTHER LIQUOR FROM THE CRYSTALLIZATION STEP MAY THEREAFTER BE ISOMERIZED TO READJUST THE PARA-XYLENE CONTNET AND THE PRODUCT OF ISOMERIZATION MAY BE RECYCLED TO THE CRYSTALLIZATION ZONE.

United States Patent Ofice 3,585,251 ALKYL TRANSFER OF ALKYL AROMATICS WITH GROUP VI-B METALS ON BORIA-ALUMINA Stephen M. Kovach and Ronald A. Kmecak, Ashland, Ky., assignors to Ashland Oil & Refining Company, Houston, Tex.

Filed Dec. 19, 1968, Ser. No. 785,231 Int. Cl. B013 11/06; C07c 3/58, 15/08 U.S. Cl. 260-672 8 Claims ABSTRACT OF THE DISCLOSURE drogen introduced at a rate of about 1 to 10 moles hydrogen per mole of hydrocarbon feed. Promoters selected from Group I, Group II, Group IV and the rare earth metals of the Periodic System can also be added to the catalyst. Deactivated catalysts may be periodically rejuvenated by discontinuing the introduction of the aromatic feed material and purging with hydrogen and the catalyst can be reactivated by calcination in an atmosphere such as air. Where toluene is the feed material, the alkyl transfer product may be distilled to separate benzene, toluene and xylenes, the toluenes may be recycled to the alkyl transfer step, the Xylenes may be crystallized to separate para-xylene from the remaining xylenes, the mother liquor from the crystallization step may thereafter be isomerized to readjust the para-xylene content and theproduct of isomerization may be recycled to the crystallization zone.

BACKGROUND OF THE INVENTION The present invention relates to a process for the catalytic conversion of hydrocarbons and, more particularly, to a process for the catalytic alkyl transfer of mono-alkyl aromatics.

Aromatic hydrocarbons, such as benzene, naphthalene, and their alkyl derivatives are important building blocks in the chemical and petrochemical industries. For example, benzene and its derivatives have numerous uses; cyclohexane is utilized in nylon production; naphthalene is utilized in the production of phthalic anhydride for alkyd resins, etc.; para-xylene can be used for the production of terephthalic acid which, in turn, is utilized in the production of synthetic resins, such as dacron, mylar, etc., etc.

For many years, the primary source of such aromatic hydrocarbons has been coal tar oils obtained by the pyrolysis of coal to produce coke. Such coal tar oils contain principally benzene, toluene, naphthalene, methylnaphthalene and para-xylene. Benzene may be produced from such oils by direct separation, such as distillation techniques, the para-xylene may be separated by crystallization, and the naphthalene fractions by direct separation techniques. Further alkyl derivatives of benzene and naphthalene can be converted to increased volumes of benzene and naphthalene by hydrodealkylation.

More recently, however, the petroleum industry has become a leading source of these aromatic hydrocarbons. The reason for this has been the availability of the catalytic reforming process in which naphthene hydrocarbons are dehydrogenated to produce a reformate rich in Patented June 15, 1971 aromatics and more efiicient processes for separating the aromatics from the reformate.

Some years ago, there was a high demand for toluene which was used in the production of TNT. This led to the building of substantial facilities for its production. However, the advent of nuclear and fusion weaponry and the use of diesel oil-ammonium nitrate explosives has left toluene in substantial over-supply, since the only major uses of toluene are as a solvent, the production of toluene diisocyanates and the production of benzene. This has resulted in extensive efforts to develop methods for con vetting toluene to benzene. One method of converting toluene to benzene is by the previously mentioned hydrodealkylation.

Dealkylation has the primary disadvantage that methane is a major product. Volume yields of benzene are therefore low and carbon deposition on the catalyst is high. The large amounts of methane, while useful as a fuel, require expensive techniques for the removal of the methane from the circulating hydrogen stream utilized in the hydrodealkylation. In addition, large quantities of hydrogen are consumed in the dealkylation process and hydrogen is often in short supply and expensive to produce. Finally, where catalysts are used in the process, carbon laydown on the catalyst is a serious problem.

A more profitable reaction for changing alkyl aromatics to other aromatic products is an alkyl transfer reaction. An alkyl transfer reaction is a process wherein alkyl groups are caused to be transferred from the nuclear carbon atoms of one aromatic molecule to the nuclear carbon atoms of another aromatic molecule. By way of example, an aromatic hydrocarbon molecule containing one nuclear alkyl substituent, such as toluene, may be treated by disproportionation to produce an aromatic hydrocarbon with no alkyl substituents, namely, benzene, and aromatic hydrocarbon molecules with two nuclear alkyl substituents, namely xylenes. Similarly, product ratios may be shifted by transalkylation of xylene and benzene to toluene. Such alkyl transfer reactions have distinct advantages: methane is not produced, but instead, valuable aromatic hydrocarbons are produced in addition to the desired aromatic hydrocarbon. As a result, there is very little loss of product in alkyl transfer as opposed to hydrodealkylation.

Alkyl transfers may be carried out thermally. However, thermal alkyl transfer results in demethylation due to cracking and hydrogenation, ultimately resulting in low yields of desired aromatics. On the other hand, catalytic alkyl transfer has not been highly successful since it re quires an active, rugged, acidic catalyst. Typical catalysts are solid oxides, such as silica-alumina, silica-magnesium, etc. These materials, however, are not active enough to promote disproportionation at high conversion rates. In addition, as is the case in hydrodealkylation, carbon deposition on the catalyst and its afiect on catalyst activity with time is a severe problem.

It is therefore an object of the present invention to provide an improved process for the conversion of monoalkyl aromatics. Another object of the present invention is to provide an improved process for the alkyl transfer of monoalkyl aromatics. Yet another object of the present invention is to provide an improved process for the disproportionation of toluene to produce benzene and xylenes. Another and further object of the present invention is to provide an improved process for the disproportionation of mono-alkyl aromatics which utilizes a novel catalyst system. Another object of the present invention is to provide an improved process for the disproportionation of mono-alkyl aromatics with a catalyst system resistant to carbon laydown. A further object of the present invention is to provide an improved process for the catalytic disproportionation of mono-alkyl aromatics utilizing a Group VIB metal and boria on an alumina base. A further object of the present invention is to provide an improved process for the disproportionation of mono-alkly aromatics utilizing critical conditions of temperature and pressure which produce maximum disproportionation and conversion of one aromatic to another. Still another object of the present invention is to provide an improved process for the conversion of toluene to benzene and xylenes and conversion of metaand ortho-xylenes to additional para-xylene. These and other objects and advantages of the present invention will be apparent from the following detailed description.

SUMMARY OF THE INVENTION Briefly, in accordance with the present invention, alkyl transfer of alkyl aromatics comprises contacting an alkyl aromatic feed material with a catalyst comprising a metal of Group VI-B of the Periodic System and boria deposited on an alumina base. Further improvement of the catalyst is obtained by adding a Group I, Group II, Group IV, a rare earth metal or mixtures thereof. Further improvements of the process are obtained by maintaining the temperature between about 800 and 1100 F. and the pressure between about and 2000 p.s.i.g. Where toluene is the feed, additional para-xylene is produced by isomerizing orthoand meta-xylenes.

BRIEF DESCRIPTION OF THE DRAWINGS The drawing shows a flow diagram of a process system in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION Mono-alkyl aromatic feed materials for use in accordance with the present invention can be any mono-alkyl aromatic having a transferable alkyl group. Primary materials are substantially pure mono-alkyl aromatics, mixtures of such mono-alkyl aromatic hydrocarbons, or hydrocarbon fractions rich in such mono-alkyl aromatic hydrocarbons. Such feeds include monoand di-aromatics, such as mono-alkyl benzene and mono-alkyl naphthalene. Preferably, the alkyl group should contain no more than about 4 carbon atoms. A preferred feed in accordance with the present invention is toluene. Accordingly, disproportionation of toluene will be referred to hereinafter in the detailed description.

The process of the present invention must be conducted at a temperature between about 800 and 1100 F., and preferably between 850 and 1000 F. It has been found in accordance with the present invention that below this temperature range, substantially decreased conversion occurs due to hydrogenation. On the other hand, when operating above this temperature range, thermal demethylation occurs. Ideally, the temperature is 950- 1000 F. The pressure utilized in accordance with the present invention has also been determined to be a critical factor. Accordingly, the process should be carried out between about 0 and 2000 p.s.i.g., and preferably between 400 to 1000 p.s.i.g. It has been found that below the desired pressure range, conversion is low and the aromaticity of the product is high. On the other hand, at higher pressures, conversion is high, but liquid recoveries are low due to hydrogenation and hydrocracking. A liquid hourly space velocity between about 0.1 and 10, and preferably between 0.25 and 1.0, should be utilized and a hydrogen-to-hydrocarbon mole ratio between about 1 and to 1 and 2 and 4 to 1 is desired.

The high severity conditions required to obtain disproportionation of mono-alkyl aromatics, particularly the disproportionation of toluene, has been found to lead to catalyst deactivation due to selective adsorption and condensation of aromatics on the catalyst surface and carbon laydown on the catalyst. It was found that the condensation and adsorption of aromatics on the catalyst is a temporary poison and that this condition can be alleviated by utilizing high hydrogen partial pressures. In addition, this temporary deactivation of the catalyst can be overcome to completely rejuvenate the catalyst to near virgin activity by hydrogen-purging of the catalyst in the absence of aromatic hydrocarbon feed. While coke or carbon deposition on the catalyst is a permanent poison, it has been found, in accordance with the present invention, that carbon laydown can be decreased by utilizing the catalysts of the present invention. Further, it was found that when these catalysts become deactivated by carbon laydown, they can be restored to near virgin activity by regeneration in air.

The data below illustrate the effectiveness of hydrogen purging in maintaining catalyst activity over a long period of time.

5% CI'203'10% B203Al203 1000 F., 800 p.s.i.g., 1 LHSV, 3/1 H /H'C H purge between cycles (3-hour process1-hour purge) Conversion weight Cycle number: percent feed Longer purging periods than indicated above produce even better results.

The catalyst employed in accordance with the present invention is a multi-faceted cure for many of the ills of catalytic disproportionation reactions. First, it has been found that foria deposited on an alumina base is vastly superior to a silica-alumina base catalyst; the former consistently proving to be twice as active as the latter. The amount of boria deposited on the alumina may vary between about 5 and 25% by weight of the finished catalyst. The alumina is preferably a gamma alumina. Such gamma aluminas are very stable up to temperatures of about 1800 F. One such alumina, Boehmite, may be prepared in a variety of ways, one of the simplest being the addition of ammonium hydroxide to a water solution of aluminum chloride. The material originally precipitated is an amorphous alumina flock which rapidly grows to crystal size yielding crystalline Boehmite. Aging of Boehmite in ammonium hydroxide solution transforms the Boehmite first to the meta-stable Bayerite and finally to the highly stable Gibbsite. The Bayerite is preferred and may be in its betaor etaform.

The active Group VIB metal added to the boriaalumina includes chromium, molybdenum and tungsten, preferably in their oxide form. The Group VI-B metal should be present on the finished catalyst in amounts between about 1 and 20% by weight and preferably about 4 and 6% by weight.

It has also been found that conversion may be further improved and, more significantly, carbon laydown on the catalyst may be further reduced by the addition thereto of a promoter. Such promoters may be selected from Group I of the Periodic System, such as potassium, rubidium, cesium, etc., Group II of the Periodic System, such as calcium, magnesium, strontium, etc., a rare earth metal of the Periodic System, such as cerium, thorium, etc., a Group IV metal of the Periodic System such as tin or lead, or mixtures of these, and particularly mixtures of a Group IV metal with one of the other groups mentioned. The promoters are preferably in their oxide form and are present in amount of about 1 to 15% by weight based on the weight of the finished catalyst.

The catalysts may be prepared by techniques well known in the art. For example, such preparation may include coprecipitation or impregnation techniques. One can employ extrudates or pellets for impregnation or powders followed by pelletization or extrusion to yield the finished catalyst. When employing impregnation techniques, the metals may be added to the base singly or in combination, utilizing water soluble salts such as borates, boric acid, halides, nitrates, sulfates, acetates, chromic acid, etc. Easily hydrolyzed salts can be kept in solution without decomposition by employing the appropriate inorganic acids. Well-known procedures for drying and calcination of the catalyst will also be employed. For example, vacuum drying at a temperature of about 250 F. and calcination in oxidative, neutral or reductive atmosphere, utilizing a calcination temperature of about 800 to 1200 F. can be practiced.

Preparation of a specific catalyst is illustrated below.

To 200 ml. of distilled water was added 25 g. of chromium sulfate and 31 g. of boric acid. The solution was heated until all the metal salts went into solution. This hot solution was added slowly to 250 ml. of bayerite alumina and allowed to stand for 15 minutes. At the end of this period, the liquid was decanted from the impregnated alumina. The resulting catalyst was dried at 250 F. for 1 hour in a vacuum oven and then calcined in a muffle furnace at 950 F. in air for 16 hours. This yields the following composition:

% CI203'10% B203'A1203 TABLE I Catalyst 5% Cl'zOg-SiOz-AlzOa 5% C1'zO3-l0% B2O -A1zO Temperature, F 1, 000 1, 050 900 950 1, 000 Pressure, p.s.i.g 400 400 400 400 400 LHSV O. 5 0. 5 0. 5 0. 5 0. 5 Liquid recovery. 97 94 97 97 95 Benzene-xylene,

weight percent feed 18 28 15 25 34 While the previous Table I also indicated the criticality of temperature in the reaction of the present application when utilizing the chromia-boria-alumina catalyst, the following Table II illustrates the eiIect of contact time on the reaction.

TABLE II 5% 01203, Bach-A1203 Temperature, F 1, 000 Pressure, p s i g 800 LHSV 0. 5 Liquid recovery 89 Benzene-xylene, weight percent ieed 12 34 40 Based on the above, it is obvious that higher temperatures and higher pressures are required to obtain high conversion of toluene to benezene and Xylene. Increased contact time is not only obtained through the low liquid hourly space velocity, but also through the use of increased pressures.

A third variable in the present reaction is carbon laydown on the catalyst and its effect on catalyst activity with time. Carbon laydown can be controlled to a certain extent by the addition of hydrogen to the system. In order to make the hydrogen most effective in this system, the catalyst must be one that does not possess aromatic nuclei hydrogenation activity. The following Table III shows the eifect of hydrogen partial pressure in the presence of a chromia-boria-alumina catalyst.

It is to be noted from Table III that low hydrogen-tohydrocarbon ratio gives a higher conversion but a faster rate of decline of catalyst activity. However, increasing the hydrogen ratio to about 2 to 3 moles per mole of hydrocarbon feed decreases the conversion only slightly but the catalyst activity decline has been improved by a factor of about 3 to 4 times.

The following Table IV compares a chromia-boriaalumina catalyst with bOria-alumina having no active metal and with boria-alumina utilizing platinum as an active metal. Platinum is an eflfective metal for the present reaction, but it is to be noted that at comparable conditions, liquid recoveries with platinum were lower than with chromia, naphthene production was higher, and the rate of change of catalyst activity is higher. This difference is believed to be due to the aromatic nuclei hydrogenation activity of Pt.

In addition to the factors previously discussed and illustrated, the present reaction is critically dependent upon the concentration of the Group VI-B metal on the catalyst. Table V below shows the effects of utilizing a catalyst containing 5% chromia as opposed to one containing 10% chromia.

TABLE V [Conditionsz 1,000 F., 800 p.s.i.g., 1 LHSV, 3/1 HQ] Catalyst 10% (H203, 5% C1103, zoalzoa Enos-A120 Conversion, weight percent feed 35. 3 41. 7 A conversron, weight percent per hour 6. 0 1. 0

Table VI below shows the elfects of adding a promoter to the Group VIB metal catalyst:

TABLE VI Catalyst 5Cl'zO -2SHO, 5ClOz-ZK20, 501303-2080 10B203-A1z03 IOB203-A1z03 IOB O -AlqO 3 Feed Toluene Toluene Toluene Conditions:

Temp., F- 1,000 1, 000 1, 000 Pressure, p s 800 800 800 LHSV 0. 5 0. 5 0. 5 HQIHCU 3/1 3/1 3/1 Recovery, v01. percent 88 88 88 Toluene conversion 46 34 36 Product distribution:

Benzene 3.6 2. 2 3. 9 27. 9 21. 8 18. 8 61. 2 75. 5 72. 4 7. 0 0. 5 4. 9 0. 3

In accordance with the present invention, an integrated process for the process for the production of benzene and para-Xylene from toluene can be carried out with resultant high yields of these two valuable products. This process is best described by reference to the drawing.

In accordance with the drawing, toluene is introduced to the system through line 10, hydrogen is added through line 12 and these materials are passed over the catalyst of the present invention in the disproportionation reactor 14. The efiiuent passing through line 16 is passed to a flash drum for the removal of hydrogen and any light gases produced. These materials are discharged through line 18. Since little or or no demethanation occurs, the hydrogen is substantially pure and may be recycled to the disproportionation reaction without further treatment. However, in some instances, further purification of the hydrogen is necessary before recycle or reuse. The liquid product passes through line 20 to a first distillation unit 22. In distillation unit 22, benzene is recovered as an overhead through line 24. The bottoms product from distillation unit 22 passes through line 26 to a second distillation unit 28. In distillation unit 28, toluene is removed as an overhead product and recycled to the disproportionation section through line 30. The bottoms product from distillation unit 28 is a mixture of xylenes which is discharged through line 32. This product may be withdrawn, as such, through line 34. Preferably, however, the xylene product is passed through line 36 to crystallization unit 38. In crystallization unit 38, para-xylene is selectively removed and withdrawn through line 40. The mother liquor from the crystallization section is passed through line 42 to an isomerization unit 44. Hydrogen is added through line 46. In the isomerization unit 44, the equilibrium concentration of para- Xylene is re-established and the material may then be recycled through line 48 to crystallization unit 38 for further para-xylene separation.

The isomerization reaction should be carried out under more mild conditions than the disproportionation. Catalysts useful in the disproportionation reaction might also be used in the isomerization or conventional catalysts, such as, platinum on silica-alumina, can be used. The isomerization may be carried out at temperatures of about 500 to 900 F., and preferably 550 to 650 F., pressures of 50 to 2000 p.s.i.g., and preferably 300 to 600 p.s.i.g., at a liquid hourly space velocity of 0.1 to 10, and utilizing a hydrogen-to-hydrocarbon mole ratio between about 1 and 20 to 1.

When reference is made herein to the Periodic System of Elements, the particular groupings referred to are as set forth in the Periodic Chart of the Elements in The Merck Index, Seventh Edition, Merck & Co., Inc., 1960.

The term alkyl transfer of alkyl aromatics as used herein is meant to include disproportionation and transalkylation. Disproportionation, in turn, is meant to include 'conversion of two moles of a single aromatic, such as toluene, to one mole each of two different aromatics, 'Such as xylenes and benzene. Transalkylation is meant to include conversion of one mole each of two different aromatics, such as xylenes and benzene, to one mole of a single difierent aromatic such as toluene. The alkyl transfer defined above is also be disinguished from isomerization where there is no transfer of alkyl groups from one molecule to another but simply a shifting of alkyl group around the aromatic ring, such as isomerization of xylenes, or rupture of the ring or the alkyl side chain and rearrangement of split-off carbon atoms on the same molecule. The alkyl transfer is also to be distinguished from a hydrogen transfer reaction, such as the hydrogenation of aromatics, the dehydrogenation of cycloparaffins and like reactions.

We claim:

1. A process for alkyl transfer of monoalkyl aromatics; comprising, contacting an alkyl aromatic feed material with a catalyst comprising about 1 to 20% by weight of a Group VI-B metal of the Periodic System, about 1 to 15% of a metal selected from the group consisting of Group I, Group II, Group IV and the rare earth metals of the Periodic System and mitxures thereof and about 5 to 20% by weight of boria deposited on an alumina base, under condltlons suflicient to cause alkyl transfer of said alkyl aromatics.

2. A process in accordance with claim 1 wherein the group VI-B metal is chromium.

3. A process in accordance with claim 1 wherein the feed material contains substantial volumes of toluene.

4. A process in accordance with claim 3 wherein unconverted toluene is separated from the alkyl transfer products and said unconverted toluene is recycled to the disproportionation step.

5. A process in accordance with claim 3 wherein Xylenes are separated from the alkyl transfer product and para-xylene is separated from said Xylenes.

6. A process in accordance with claim 5 wherein the xylenes remaining after the separation of para-xylene are subjected to isomerization conditions sufficient to produce additional para-xylene.

7. A process in accordance with claim 3 wherein the flow of feed material through the catalyst is interrupted periodically and the flow of hydrogen is continued for a. time sufiicient to reactivate the catalyst.

8. A process in accordance with claim 3 wherein the flow of feed material and hydrogen through the catalyst is discontinued and the catalyst is calcined in air under conditions sufficient to reactivate the catalyst.

References Cited UNITED STATES PATENTS 2,795,629 6/ 1957 Boedeker 260-668 3,260,764 7/ 1966 Kovach et a1 260672 3,476,821 11/1969 Brandenburg et a1. 260672 3,197,518 7/1965 Chapman et al. 260668 DELBERT E. GANTZ, Primary Examiner G. E. SCHMITKONS, Assistant Examiner U.S. Cl. X.R.

252-432; 260668A, 672T, 674A 

